Category Archives: Traffic Signals

The caption with the picture below in the Starts and Stops column of the Metro section of the June 17, 2012 Boston Globe reads:

Cyclists stopped for a red light in the “bike box” on Commonwealth Avenue in the Back Bay. They provide the cyclist a safe space to wait ahead of cars at traffic signals.

Photo which appeared in the Boston Globe Metro section, June 9, 2012

(The Globe story may be behind a paywall, but you can probably access it through a public library’s Web site using your library card number.)

The smiling cyclists show that this is a posed photo; the photographer evidently only thought of the large puddle in the foreground as an artistic touch. How about the car encroaching into the bike box in the background?

Well, yes, OK, waiting in the bike box might be safe — drivers are unlikely to encroach on a cyclist who is already waiting in the bike box. The problem is with getting into the bike box. The Globe columnist, Eric Moskowitz, never considered that bicyclists approaching the bike box on a red light are encouraged to swerve sharply left across multiple lanes of motor vehicles, with no way to know when the light will turn green. A waiting motorist will not see the swerving cyclist if looking to the left for traffic at the wrong moment. A tall vehicle in one lane will conceal the cyclist from a driver waiting in the next lane.

Portland, Oregon has hosted a study of bike boxes, which found that this is actually a rare problem in Portland, because cyclists are smart enough not to swerve into the bike box. Instead, if the light is red, they wait at the right curb, blocking other cyclists behind them. I saw the same thing on Commonwealth Avenue. As I said before, the Globe photo is posed.

But on the green light, there’s another problem. Bike boxes and the bike lanes which lead to them invite cyclists to overtake waiting motor vehicles on the right, risking getting struck by a right-turning vehicle. A bicyclist was right-hooked and killed in Portland, Oregon, on May 16, 2012 but apparently that news didn’t reach the Globe’s columnist, or didn’t make an impression on him. Now a letter from the City of Portland is conceding that car-bike crashes have increased at some of the intersections where bike boxes were installed. So much for the Globe’s assertion of safety.

Conscientious bicycling advocates have been warning about the “right hook” problem for decades, based on the difficulty which motorists have in looking into their right rear blindspot, while also checking the intersection ahead.

Swerving across is illegal too: here’s the Massachusetts law, in Chapter 89, Section 4A. It applies to bicyclists, the same as other drivers. Every state has a similar law.

When any way has been divided into lanes, the driver of a vehicle shall so drive that the vehicle shall be entirely within a single lane, and he shall not move from the lane in which he is driving until he has first ascertained if such movement can be made with safety.

Bicycling advocates, planners and government officials who promote bike boxes have simply chosen to pretend that this traffic law doesn’t exist, or can be ignored. Same for the limits of human abilities.

Now, I wouldn’t be fair in making this criticism if I didn’t suggest alternatives.

The one I favor is for cyclists to merge before reaching the intersection. That can be facilitated by signal timing at the previous intersection to allow cyclists to merge across when motor traffic is stopped, and a clear lane into which to merge.

These measures will, however, result in more delay, for both cyclists and motorists.

It may be more practical just to designate another street as the one for through bicycle traffic, My favorite suggestion at this Back Bay location would be Newbury street, configured as a two-way bicycle boulevard with a bridge over the Muddy River to connect it with the Fenway area.

A reader pointed me to a news story on the politiken.dk blog about the Copenhagen/Albertslund “bicycle superhighway” which is getting attention and publicity. The reader’s comments on my previous post read:

Yeah, its kind of joke, but to be fair they are not called superhighways in Danish but Super bicycle tracks, and even then most agree that they are not really that super. There is a video of the entire route here if you scroll down a bit:

The two next ones which will open are another story though, as they mostly have their own right of way, and use viaducts or bridges to cross streets.

So, better things may be on their way, but…I ran the article through the Google translator, and it appears in the link below in (sort of) English. The page includes the sped-up video of the entire route.

Here’s the video — warning, Shell diesel fuel ad at start, and you can only stop the video when you click on it, see the ad again and click on it to open a bigger ad! This workaround was needed to make the video visible on this page.

The one unifying factor of this route is an orange line painted lengthwise to identify it. The first part of the route is relatively tame. Barriers, unprotected intersections and other hazards pile up near the end.

Some representative quotes (I’ve translated from Googlish to English, thanks to an online dictionary and my knowledge of the neighbor language, German.):

From the article:

“I did not expect that I just had to detour on ordinary roads in residential neighborhoods. I did not see much of the green wave that is supposed to be in town. I do not think you can call it a super bike path,” the [politiken dk test rider] concluded.

From comments on the article:

- The section of tunnel under Motorring 3 is dark and miserably lighted. There are many riding schools (which, incidentally, should be forced to close and move out into a rural area!). The tunnel is usually filled with horse s***, and because you can not see in these tunnels due to poor lighting, you can only hope that you do not ride through any of it.

*****

- In the westbound direction, at the pitch-dark tunnels, you have to negotiate two sets of barriers. The point of these, other than to impede traffic, I do not know. But when you have to use all your mental energy to get through these, they constitute more of a hazard than a safety precaution.

*****

I have commuted between Roskilde and the northwest part of Copenhagen 2-3 times a week on a recumbent trike with an electric assist motor for 6 months (http://ing.dk/blogs/pedalbilen). When I used the “super path” the trip was about 3 km and 15 minutes longer. Especially the part of the route in Albertslund is very indirect and inconvenient. There are detours, barriers and ramps in most places, and it will for example not be possible to ride in a velomobile, as far as I can judge. The new route is comfortable and free of exhaust, but as commuter route it gets a failing grade compared with Roskildevej [a parallel, 4-lane divided but not limited-access highway with one-way sidepaths].

*****

- I didn’t see anything which shows that cyclists have priority over the other traffic. Unfortunately, the only thing new that I see is approximately 100 meters of new asphalt in two places near Rødovre, so that it is easy going. There are simply no real improvements for cyclists in relation to other road users! You can still find barriers, sharp turns, bumps and traffic lights. Why is there no new cycle path, e.g. along the western forest road, so you do not have to drive through neighborhoods with pedestrians and children playing? Why are barriers not turned 90 degrees, so users of the route have right of way?

Even if there were brand new asphalt on the entire route it would never merit the title “super”. Only when a route enables more or less continuous travel at high average speed (which motorists know from motorways) does it, in my opinion, deserve the massive marketing it is currently getting.

*****

…Bus passengers cross the bikeway. It seems quite unreasonable that there are no islands at bus stops where passengers have to wait when they get on and off. Thus cyclists must stop, and so, so much for the “super bike path”.

An intersection with light rail, motor vehicles and bike lanes requires bicyclists to cross from one side to the other of a multi-lane street, resulting in delays of 2 to 3 minutes. Alternative solutions are described.

New York bicycling advocate Steve Faust has stated that some ways of accommodating bicycling do not “scale up” — that is, they work with small numbers of cyclists, but less well with larger numbers.

His central complaint is that use of roadways with no special bicycle facilities, according to the conventional rules of the road, does not scale up well.

I might put that a bit differently. After all, more cyclists need more room. Mass rides such as New York’s own 5-Borough Tour avoid special bicycle facilities and occupy the entire width of Manhattan’s multi-lane avenues. Motor vehicles are excluded while these rides pass through. Interaction within the group of many thousands of cyclists is for the most part according to the conventional rules of the road, and falls short only in that many of the participants are inexperienced.

On roadways carrying both cyclists and motorists, cyclists inconvenience motorists when the motor traffic could go faster — that is, when there are many cyclists and few enough motorists that they could travel unimpeded, if not for the cyclists. Motorists inconvenience cyclists when motor traffic is congested, and stopped or traveling slower than cyclists might want to go. Level of service always declines as a road becomes more congested, and it declines faster when vehicles have differing speed capabilities.

On the other hand, there also are situations in which operation as intended does not scale down to smaller numbers.

Motorists are more likely, for example, to yield to a crowd of pedestrians than to a single pedestrian.

Another example is the leading pedestrian interval: the walk signal goes on a couple of seconds before motorists get the green light. The leading pedestrian interval is intended to get pedestrians moving out into the intersection before motor traffic can begin to turn across a crosswalk, encouraging motorists to yield to the pedestrians. The same approach is used sometimes on bicycle facilities, for example on the Boulevard de Maisonneuve bicycle sidepath in Montréal, Québec, Canada. But a leading interval only works if there is someone waiting to cross when the signal changes. With smaller numbers, so the first pedestrian or bicyclist reaches the crossing after the motorists get their green light, the leading interval’s only achievement is slightly to reduce the capacity of the intersection.

The same issue can occur with any “conflict zone” with poor visibility as users approach, including the “bike box” or bicycle waiting area ahead of the stop line for motorists at an intersection. Once one cyclist is in a “bike box”, a motorist is unlikely to move forward, because that would require running over the cyclist. Therefore, the bike box is then safe for the entry of other cyclists, at least into the same lane in which the first cyclist is waiting.

The”bike box” works as intended when there are large numbers of cyclists so the first one arrives well before the traffic signal turns green.

If there are few cyclists, so the first one is likely to arrive just as the traffic signal turns green, then there is the potential for a right-hook collision, or a motorist’s colliding with a cyclist swerving into the bike box.

Safety requires that there be enough cyclists that early-arriving ones block the way of motorists, or at least alert the motorists that others may arrive. This safety factor does not scale down to small numbers.

Research in Portland, Oregon shows that only 5% of bicyclists swerve into the bike box when they are first to arrive; about 35% if they arrive later. The reluctance of the first-arriving cyclist reflects risk avoidance to some extent, due to not knowing when the traffic signal will change, but also that the swerve lengthens the cyclist’s trip — none of the Portland bike boxes are designated for left turns. The later-arriving cyclists are to some degree protected by the arrival of the first one, but also they either have to wait behind or move over to the left of that cyclist, into the bike box.

“Safety in numbers” claims become rather interesting when such issues are considered.

The design challenge is to achieve efficiency and safety of all travelers, regardless of whether numbers are large or small.

My 10-year-old son Jacob’s school year hadn’t started yet on that brilliant clear September morning. My wife had left for work. I had a business appointment in Belmont, a few miles from our home. Jacob and I set out for Belmont around 9 AM on our tandem bicycle. There wouldn’t be much traffic; the morning rush hour had passed. I had 30 years of safe cycling in Boston-area traffic, and little concern about our completing the trip safely.

Through an open car window I heard a word or two…Boeing 757 airliner…I got a spooky feeling. No radios were playing music or the boom chicka chicka booom chicka rant (expletive) rant rant rant which passes as music. I’d gladly have had that instead, thank you.

Jacob and I were turning left from where the white car is in the picture. Note the hedge on the far left corner.

Jacob and I were waiting just to the right of the double yellow line at a red light, to turn left from Waverley Street onto Thomas Street, near Belmont Center. Both streets are relatively narrow two-lane, two-way streets with light traffic. My left arm was extended in a very clear left turn signal. The traffic light turned green, and just as I began to swing the tandem to the left, I heard “beep beep” of a car horn behind us. A small gray sedan going about 30 mph passed us on the left side of the street, an insanely hazardous maneuver not only because of the possible collision with us, but also because someone might be making a right turn on red from Thomas Street, and would of course be looking the other way for traffic. A hedge blocked the view around the corner, too.

Jacob and I rode the couple of hundred yards to the Belmont police station and spoke with Officer MacEachern, who was very supportive and professional. I didn’t get a look at the driver, but Jacob did, and identified her as a woman with brown or black hair (dyed?) and, he thought, wearing glasses. We reported the license number of the car, 350 JBA. The officer verified that it was for a gray Toyota Corolla sedan, and told us that it belonged to a 75 year old woman who lived in Belmont.

Officer MacEachern said that his department unfortunately could not issue a citation, since an officer had not seen the incident, but that he would go have a talk with the driver and that it might be possible to pull her license if she is incompetent.

I can understand being termporarily out of one’s mind. One time I was injured but felt no pain, even ignored the grating of bone on bone, just had to get — home. Once a truck strayed into my lane and sideswiped my car. Only my quick swerve avoided a head-on crash. I was going to turn around and chase after the truck except that my wife calmed me down.

Maybe the driver was the daughter of the car’s owner, who was in turmoil…maybe she had children at home and hoped to spend her final minutes with them.

Maybe she had someone on one of those planes — two flew from Boston — or in one of those buildings.

Jacob and I rode on to the home of my client where I shot the photo below.

Television news coverage of the September 11 attack on the Pentagon.

And that is how my son and I might have been killed on September 11 2001, not by Al Queda fanatics in red-hot hatred, though very likely through a ripple effect of their apocalyptic, ruinous madness onto someone I would probably have been happy to have as my next-door neighbor.

I went back a few weeks later and took the picture of the intersection, so I could include it in what is mercifully the story of a non-event and gets news coverage only here. Even if Jacob and I had been killed or gone to the hospital, our crash would have gotten even less attention than bicycle crashes usually do, on that day.

A take-away? Every variety of opinion about Al Qaeda has probably already been expressed so I’ll only say this: adrenalin is a powerful mind-altering drug. It is available only by prescription except when you make it yourself, and because you do, police can’t cite you for it after a crash. But still, please think twice…

The video embedded below documents a bicycle ride by the author and two companions in Washington, DC where special bicycle signals and road markings have been installed to establish continuity of New Hampshire Avenue at its intersection with U Street and 16th Street.

The video tells the story of an unusual pass through the intersection — the signal actuator somehow didn’t give us the green on the first go-round. It did on other passes.

Aside from that one quirk, the video shows the usual signal timing (though it might be different at another time of day). The most significant finding is the short time to get into the “bike box” which is located ahead of motorists’ advanced stop line. There wouldn’t be be enough time for any large number of bicyclists who had been waiting for the signal.

Several additional issues remain to be shown in other videos. Here is a preview of what they will show:

The bike boxes are very small, also a problem with large numbers of bicyclists.

Motorist encroachment into the bike boxes is endemic.

Some motorists misunderstand the installation to the degree that they become angry at bicyclists who use it as intended. We were buzzed on one pass by someone driving a big black SUV.

Probably 90 percent of cyclists passing through this intersection on New Hampshire Avenue do not use the facility as intended. Rather, most use the crosswalks — commonly going around clockwise. With the option to go either clockwise or counterclockwise, bicyclists can start across on either pedestrian phase, rather than having to wait through as much as an entire signal cycle for the special bicycle phase.

Before I actually rode through this installation, I thought that it was a particularly good one. I have a blog post expressing that opinion. That post also gives a detailed description of the installation and its history.

The installation doesn’t violate any principles of traffic theory. In particular, when used as intended, it doesn’t place bicyclists and motorists out of sight of each other and on a collision course. But as Yogi Berra, or maybe Albert Einstein, memorably said, “[I]n theory, theory and practice are the same. In practice, they are not.” This installation makes the intended movements unattractive due to the short timing and tight space. The installation also makes bicyclists wait much longer to take the intended routes than to take others — which don’t even necessarily require illegal movements. As such, this installation unfortunately is not, in my opinion, successful.

On March 14, 2011, I attended a meeting in Brookline, Massachusetts, a suburb of Boston. At that meeting, Dr. Peter Furth of Northeastern University, and some of his students, gave a presentation on proposals for street reconstruction and bikeways in the southern part of Brookline.

Proposed replacement of two two-lane one-way roadways with shoulders by a narrowed two-way roadway without shoulders and an adjacent multi-use path; Newton Street (up the embankment at the left) to provide local access only.

Most of the streets in the project area are fine for reasonably competent adult and teen cyclists to share with motorists, though one street, Hammond Street, is much less so, with its heavy traffic, four narrow lanes and no shoulders. I do agree with a premise of the presentation, that bicycling conditions could be improved, but I suggest different treatments, such as conversion from four lanes to three, with a center turn lane which becomes a median at crosswalks, also freeing up room at the sides of the roadway for motorists to overtake bicyclists.

Please read through this introduction before looking through the photo album I have posted with images from the meeting presentation, and drawings which were taped up in the meeting room.

My Concerns with the Proposal

Generally, I am concerned with the hazards and delays — and in winter, complete lack of service — which the proposal introduces for bicyclists, and with the blockages and longer trips it introduces for motorists, resulting in delay and in increased fuel use and air pollution. Some specifics:

The proposed system of narrow two-lane streets and segregation of bicyclists onto parallel paths makes no provision for the foreseeable increasing diversity of vehicle types and speeds as fuel prices rise. This trend can only be managed efficiently and flexibly with streets that are wide enough to allow overtaking.

Proposed paths are on the opposite side of the street from most trip generators, and many movements would require bicyclists to travel in crosswalks, imposing delay, inconvenience and risk for bicycle travel on the proposed route and on others which cross it. Small children would not safely be able to use the proposed routes unless accompanied by an adult, same as at present.

The proposal would further degrade or eliminate bicycle access in winter, because of the proposed 22-foot roadway width, and because the proposed parallel paths could not be kept ice-free even if they are plowed.

Though intersections are key to maintaining traffic flow, the proposal puts forward an incomplete design for most intersections and no design for some of them.

The narrowed streets provide no accommodation for a vehicle which must stop while preparing to turn left into a driveway, or to make a delivery, or to pick up or discharge passengers, for bicyclists who must travel in the street to reach a destination on the side opposite the path, or for bicyclists who wish to travel faster than the path allows.

The proposal includes no discussion of improvements to public transportation, which would be key to reductions in congestion and fossil fuel use. There are no bus turnouts, although Clyde and Lee Streets are on an MBTA bus line, school buses use the streets in the project area, and additional bus routes are foreseeable.

The proposal includes a 12-foot-wide service road along Clyde and Lee Street, part of which is to carry two-way motor traffic along with bicycle and pedestrian traffic. At times of heavy use, 12 feet on the Minuteman Bikeway in Boston’s northwestern suburbs is inadequate with only pedestrian and bicycle traffic. 12 feet is not wide enough to allow one motor vehicle to pass another. Larger service vehicles (moving vans, garbage trucks etc.) could access parts of this service road only by backing in, or by driving up a curb and across landscaped areas. These vehicles would completely block other motorized traffic on the service road.

The proposal is expensive because it requires tearing up every one of the streets in order to narrow it, not only construction of a parallel path.

Confusion in the Presentation

There also was confusion in the presentation:

North is confused with south, or east with west in the captions to several photos. Due to the confusion, some photos show a path on the opposite side of the street from where the plan drawings place it.

Some items in the illustrations are out of proportion. In one illustration (the one near the top in this post), a two-lane arterial street is only 10 feet wide, based on the height of a Segway rider on an adjacent path, or else the Segway rider is 12 feet tall, riding a giant Segway.

Other details are inconsistent, for example, showing a sidewalk in a plan drawing, but no sidewalk in an image illustrating the same location.

There is confusion about location of some of the photos. Some cross-section drawings are shown without identification of the location in the plans. The location of one cross section is misidentified.

The proposal makes unsupportable claims about safety.

There also is an ethical issue: in their presentation, the students have appropriated a number of Google Street View images without attribution — a violation of copyright and of academic ethics. (Furth’s students also plagiarized photos from my own Web site for a different presentation, but I digress.)

Overview and Conclusions

The proposal generally attempts to make bicycle travel a safe option for children and for people who are new to bicycling. It fails to accomplish that, due to problems with access across streets to the proposed pathways. It also adds complication and delay for motorists and for the majority of existing and foreseeable bicycle users. It degrades and sometimes eliminates bicycling as an option in the winter months, and it pays no attention whatever to public transportation.

I have no objection to construction of a path in the parkland adjacent to the streets in the project area, but the proposal also works to enforce the use of the path by reducing the utility of the road network for bicyclists as well as for other users.

I do think that street improvements are desirable, and on one street (Hammond Street) a high priority to improve bicycling conditions, but these improvements can be achieved mostly through restriping, without the massive reconstruction, or rather, deconstruction, that has been proposed. This narrowing the roadways is intended to increase greenspace, and also apparently to reduce speeding, but the proposal goes way overboard in reducing capacity, convenience and flexibility. There are other options to reduce speeding, most notably enforcement and traffic-calming measures which affect speed without decreasing capacity.

The large multi-way rotary intersection of Hammond, Lagrange and Newton Streets, West Roxbury Parkway and Hammond Pond Parkway is the one place where I consider reconstruction to be a high priority.

Education also is an essential element of any attempt to make bicycling safer and a more practical option.

Larger Contextual Issues

Long-run issues of energy cost and availability raise questions about the viability of sprawled suburbs whose residents are dependent on private motor-vehicle travel.

South Brookline is more fortunate. It is a medium-density residential area of single-family homes, only about 5 miles from the Boston city center and also only a few miles from the Route 128 corridor, a major employment concentrator. Schools, places of worship, parklands and shopping are closer than that. Bicycling can and should have a role here, but for many people and many trips, it is not an option, due to age, infirmity, distance, and the need to transport passengers and goods.

South Brookline could benefit from a comprehensive transportation plan, including strengthening of public-transportation options and maintaining arterial roads with capacity for varied existing, foreseeable and unforeseen uses.

Developing such a plan requires skills, resources and time beyond what I can muster, and so I’ll not attempt that here.

In theory, there’s no difference between theory and practice, but it practice, there is.

attributed to:
Yogi Berra
Jan L. A. van de Snepscheut
Albert Einstein

An optimal traffic-signal system would never present anyone with a red light or a don’t walk signal unless there actually is interfering traffic. In theory.

In practice, though, it may be desirable to introduce some delay in order to smooth the flow of traffic — to get vehicles on board a “green wave.” Traffic engineers think in sophisticated ways about this issue, but do not have the real-world tools to resolve it. While synchronized traffic-signal systems and sensor-actuated signals already improve the situation over uncoordinated timed signals, better sensing and more sophisticated software could, at least in theory, achieve much more.

Probably the most difficult part of the problem is in sensing approaching vehicles and pedestrians far enough ahead of an intersection so signals will change as they reach the intersection. Sensors are expensive, and many more would be needed. On the other hand, in a city dotted with security cameras, the sensor data may be easier to obtain, especially if traffic control is a goal when installing the equipment.

I am emphatically not describing so-called intelligent highway systems, intended to automate driving by taking control of vehicles. The driver then supposedly becomes a passenger, free to dial the cell phone, read the newspaper, watch TV or apply makeup without concern. For automated control to work, the system must exert at least as reliable control over vehicles as attentive drivers do. More yet: car makers have huge legal problems resulting from defects that injure only a small number of customers.

Automated control presently is applied only under very restricted conditions, on airport shuttle trains and the like. Even with a great increase in sophistication, it’s hard to conceive of how automated control (other than in collision-avoidance systems) would work on any roads except limited-access highways restricted to vehicles equipped for it.

Even under these conditions, there are difficult technical problems. Collision-avoidance systems to prevent collision with large objects ahead are just beginning to be common. Avoiding debris in the road, potholes and other smaller obstacles requires sophisticated sensing which a driver routinely performs — but well beyond the abilities of automated systems.

So, I am describing not a system to take over control of vehicles, but one to improve control of traffic signals. Humans would retain the ability to prevent collisions, and malfunctioning of the system would lead only to delay, not to crashes. The system would make little difference to anyone — motorist, bicyclist or pedestrian — except to reduce pointless delay.

Will this happen? If so, when and where? One promising thought is that it can happen bit by bit, at one intersection and another, rather than all at once along an entire highway.

I am writing this post in response to comments by Mighk Wilson and Khalil Spencer on another post on this blog. They discussed the difficulty of cycling in a city with synchronized traffic signals (a “green wave”) set to a higher speed than cyclists can manage, and the potential of a slower green wave to make a street more attractive for cycling. I’d like to take a more general look at the green wave and how it affects traffic.

My understanding of the green wave is based mostly on experience. (And so, anyone who can provide more details based on theory, or can correct me, please do…)

In my high-school years, I lived and learned to drive in Baltimore, Maryland, USA, one of the first cities to implement traffic signal synchronization. I have also lived, driven and cycled in Manhattan, where most traffic lights are timed to create green waves.

A green wave can only work under a limited set of conditions. If these do not apply, then despite best efforts to time traffic lights for the smoothest possible traffic flow, a signal sequence can still appear random. Drivers have no clear strategy for avoiding red lights beyond speeding up when the next light is still green. On the other hand, when a green wave is working smoothly, drivers may feel as if a green-wave Tinkerbelle is darting along overhead and pointing her magic wand at every traffic light to turn it green.

Traffic engineers use clever math so a green wave, surprisingly, can be applied to streets heading in more than one direction the same time — though it words better if they are one-way. Heading north on Charles Street from church in downtown Baltimore, my family would get green lights for block after block, except at the few two-way streets, where all bets were off. Then as we headed into the more random street pattern at the north end of the city, we just had to take each traffic light as it was. On the other hand, traffic lights were less frequent in this less densely built-up area.

My experience was similar in Manhattan. The green wave worked smoothly on one-way streets and avenues, but when crossing two-way ones, and when driving on them, it didn’t. This obvious difference gives drivers a strong incentive to use one-way streets and avenues for through travel, where possible. Advocates of the sort who would view streets as a neighborhood resource often protest conversions of two-way streets to one-way, see for example this call to action. Traffic engineers who are concerned with the effect on congestion and crash rates have the opposite opinion — see, for example, this presentation. (I expect that the choice is not quite so stark as these two examples make it — as usual, such decisions must be made on a case-by-case basis.)

A green wave works smoothly only when there are no stop signs on a green wave street, though stop signs can be used on cross streets. Double-parked vehicles, vehicles that have entered the street and are waiting for a light to change, vehicles — including bicycles — that can’t keep up with the pace set by the signals — anything that slows traffic down or reduces the number of lanes available increases the likelihood of not keeping up with the pase set by the lights.

A green wave often encourages travel faster than the pace set by the signals. That’s because there is an advantage in racing to the front of a platoon — where each signal has just changed to green — when preparing a turn — then after turning, racing to the end of the block so as to catch the end of the green there. A driver may speed through another few blocks to get to the front of the platoon before turning again. The advantage of this tactic is quickly obvious: After turning the corner at the head of a green wave onto another green wave street, a driver will be facing a signal at the next intersection which is about to turn yellow, then red.

The typical 30-mile per hour speed limit in grid cities like Manhattan often leads to motorists’ speeds considerably in excess of that limit, and to more unpleasant conditions for bicyclists.

On the other hand, synchronizing signals to a speed more comfortable for bicycling will discourage use of a street for through motor-vehicle travel, making it more attractive for bicycling. I have ridden on a street in Saint Petersburg, Florida, with the signals synchronized to 15 miles per hour, and it achieved that goal quite well. It would have worked better if it had been one-way — it ran up a moderate slope from the waterfront, and for most bicyclists, 15 miles per hour was hard to maintain. Downhill, on the other hand, the speed setting could have been 20 miles per hour without creating difficulties for bicyclists.

Bear in mind, though, that comfortable level-ground travel speeds for bicyclists cover a 3 to 1 range , from about 25 miles per hour down to 8 miles per hour — not nearly as uniform as for motorists, even considering the issues with motorists’ speed already mentioned. A predictable increase in the volume of electrically-assisted bicycles and motor scooters will complicate the issue of speed setting even further. The advantage of a slow green wave, given these issues, is not so much to allow bicyclists to travel farther before facing a red light as to discourage use of the street for through travel by motorists.